1. Field of the Invention
The present invention relates to a rotational speed control apparatus for internal combustion engines for controlling an idling rotational speed of an internal combustion engine by driving an idling speed control valve (hereinafter referred to as an ISCV) capable of controlling an opening of a bypass bypassing a throttle valve of the internal combustion engine.
2. Description of Related Art
Conventionally, in this kind of apparatus, a reference control value of an ISCV is computed, an engine rotational speed is detected at the time of the idling operation of the engine, a feedback correction quantity for the reference control value is computed for controlling the engine rotational speed to a desired rotational speed in accordance with an engine temperature, and the ISCV is driven based on the reference control value and the feedback correction quantity.
Learning control using a learning value is performed in the feedback control operation described above. Since a deviation is caused by a fluctuation of the rotational speed when the rotational speed control is made only by the use of a reference control value, feedback control is made in order to make an engine rotational speed coincide with a desired rotational speed Ne by further correcting the deviation described above. In this case, the feedback control value is used to update a learning value. Namely, when a given number of feedback control values have been obtained, a feedback control value obtained at an appropriate timing is adopted as a learning value and the other obtained feedback control values are nullified.
When the feedback correction quantities computed in the above-described way are stabilized within a predetermined range, a feedback correction quantity in the predetermined range is successively stored and used to update an ISC learning value for use in computing a next feedback correction quantity so that the engine rotational speed may be made to coincide with the desired engine rotational speed quickly in the course of the feedback control operation.
On the other hand, it is necessary to correct the reference control value itself when travelling on a highland because the atmospheric pressure itself is lowered. Therefore, the reference control value ISC.sub.H has been computed by the following equation. EQU ISC.sub.H =ISC.sub.BASE .times.C.sub.HAC ( 1)
In the above equation, ISC.sub.BASE represents a basic air quantity which is set in accordance with an engine temperature, and C.sub.HAC represents an ISC correction quantity, which is shown as a multiplication coefficient for the basic air quantity ISC.sub.BASE. This ISC correction quantity C.sub.HAC is set beforehand as a value corresponding to an atmospheric pressure value obtained based on an experimental result or the like, as shown in FIG. 6. The ISC correction quantity C.sub.HAC is set as "1.0" at a reference altitude (lowland), and the correction quantity is made larger (namely, the multification coefficient becomes larger) as the atmospheric pressure is lowered.
Furthermore, as a system for obtaining the atmospheric pressure value while the engine-driven vehicle is travelling, there is known a system of obtaining an atmospheric pressure value through presumptive computation of the altitude by using the ratio of an intake air quantity at the reference altitude to the intake air mass flow rate obtained by mass flow rate measuring means (for example, JP-A-2-266155), and a system of performing presumptive learning of a signal of a pressure sensor as the atmospheric pressure value, when the throttle opening has a predetermined opening value or more (for example, JP-A-59-201938).
In the case of presumptively learning the atmospheric pressure value by using techniques other than that which uses the atmospheric pressure sensor, it is often the case that the condition of performing presumptive learning of the atmospheric pressure value is satisfied when the throttle opening has a predetermined value or more. According to a result of investigation made by the Applicant, in the case of the above-referred JP-A-2-266155, the relationship between the throttle opening and the intake air quantity passing through the throttle valve is not linear, and there exists a region where a variation of the intake air quantity is reduced when the throttle opening has a predetermined value or more, as shown in FIG. 7, and very stable learning can be made when presumptive learning of the atmospheric pressure is performed in this region. In the case of a practical engine-driven vehicle, the condition of effecting presumptive learning is limited to the above-mentioned region. Further, the above-referred JP-A-59201938 relates to a system of taking in a value, just when the throttle is fully opened, as an atmospheric pressure value. Therefore, the state wherein the throttle opening is fully open is a prerequisite condition for performing the atmospheric pressure presumptive learning.
In a system in which the atmospheric pressure presumptive learning is performed under the condition of a wide-open throttle valve near its fully open state, as is the case with the conventional examples described above, the throttle wide-open condition occurs frequently when ascending a slope of a mountain road. Therefore, the atmospheric pressure learning value is updated as the altitude increases while ascending a slope. Further, since the ISC atmospheric pressure correction is also made in response to the updating of the atmospheric pressure learning value, the idling rotational speed control can be made very smoothly.
However, when considering the case of descending a slope, there would be no chance of performing the atmospheric pressure learning at all or the chance of doing so would become rare, when a driver continues to drive the engine in an idle-on state or in a very small throttle opening state, for example. In this case, there occurs a state that the last learning value of the atmospheric pressure value obtained on a highland is stored, as it is, even after the vehicle has descended to a lowland.
As a result, the ISC correction quantity C.sub.HAC has an erroneous value. Namely, according to the ISC correction quantity C.sub.HAC shown in FIG. 6, the air density is lowered as the altitude increases, as described above. Therefore, correction is made in a direction of increasing the opening of the ISCV in order to maintain the idling speed constant. Since the atmospheric pressure learning is performed correctly at time of ascending a slope, the opening correction for the ISC is also made correctly, thus causing no problem. However, if the atmospheric pressure value remains as it was obtained on a highland even after the vehicle has descended to a lowland in a slope descending mode, the correction quantity of the ISC continues to have a value which has been produced in the throttle valve opening direction as described above.
Here, the operation of the conventional electronic control device poses a problem. When the reference control value ISC.sub.H is increased, the idling rotational speed tends to increase on a lowland. However, as described above, the feedback control of the ISC and the feedback correction quantity learning function act to bring the idling rotational speed near to the desired rotational speed, and, as a result, control is made to reduce the final ISC output at this time.
At this time, an increase of the reference control value ISC.sub.H and a correction of a decrease by the ISC feedback control are performed absolutely independently from each other. Accordingly, when an erroneous atmospheric pressure learning value is retained and used on a lowland, there occurs a state such that an increasing correction amount of the reference control value ISC.sub.H due to a variation of the atmospheric pressure is decreased by the ISC feedback correction quantity. Since this decrease quantity by the feedback correction quantity is gradually replaced by the ISC learning value, the atmospheric pressure learning condition is not established immediately after descending to a lowland, but a state of a highland correction caused by erroneous learning continues until the decreased quantity learning of the ISC is completed.
Thereafter, when the atmospheric pressure learning is performed, the atmospheric pressure value becomes equal to the reference altitude (lowland) value, and a highland increase amount to be added to the ISC basic flow rate also becomes zero. Namely, the control state of the ISC presents a state of a basic flow rate devoid of highland correction plus a learning value (or an ISC feedback decrease value) subjected to a reduction by a quantity corresponding to the highland increase amount described above. As a result, when this atmospheric pressure learning value is updated, the air quantity given by the final ISC output becomes insufficient, thus resulting in a reduction of the idling speed or further in an engine stall.